It is known that the transition between traction and braking can be caused by either reversing the polarities at the terminals of the armature or by reversing the armature current in which case the direction of the polarities at the terminals of the motor remains the same, as does the flux in the main terminals. To effect the transition, it is therefore necessary to reverse the armature current with respect to the flux generated by the inductor.
Traction motors are supplied with current either via a starter rheostat or via an electronic circuit which includes a chopper whose firing and excitation are controlled. Up till now, it has been necessary to verify that the motor current has been cancelled before permitting a traction-to-braking transition (or vice-versa) since the transition, effected via contactors (or thyristors inserted directly in the motor circuit) could take place only when the motor current had been cancelled. In conventional systems, the transition is effected after a delay of three or four seconds.
The aim of the present invention is to design an entirely electronic circuit for supplying electricity to a series excitation motor in which circuit an instruction for transition from traction to braking or from braking to traction is performed virtually immediately (the delay being about one millisecond).